The organic constituents of saliva—proteins and glycoproteins—are synthesized, stored, and secreted by the acinar cells. The major products are amylase (an enzyme that initiates starch digestion), lipase (important for lipid digestion), glycoprotein (mucin, which forms mucus when hydrated), and lysozyme (attacks bacterial cell walls to limit colonization of bacteria in the mouth). Although salivary amylase begins the process of digestion of carbohydrates, it is not required in healthy adults because of the excess of pancreatic amylase. Similarly the importance of lingual lipase is unclear.
Metabolism and Blood Flow of Salivary Glands
The salivary glands produce a prodigious flow of saliva. The maximal rate of saliva production in humans is about 1 mL/ min/g of gland; thus at this rate, the glands are producing their own weight in saliva each minute. Salivary glands have a high rate of metabolism and high blood flow; both are proportional to the rate of saliva formation. Blood flow to maximally secreting salivary glands is approximately 10 times that of an equal mass of actively contracting skeletal muscle. Stimulation of the parasympathetic nerves to salivary glands increases blood flow by dilating the vasculature of the glands. Vasoactive intestinal polypeptide (VIP) and acetylcholine are released from parasympathetic nerve terminals in the salivary glands and are vasodilatory during secretion.
Regulation of Salivary Secretion
Control of salivary secretion is exclusively neural. In contrast, control of most other GI secretions is primarily hormonal. Salivary secretion is stimulated by both the sympathetic and parasympathetic subdivisions of the autonomic nervous system. Excitation of either sympathetic or parasympathetic nerves to the salivary glands stimulates salivary secretion. Primary physiological control of the salivary glands during the response to a meal is by the parasympathetic nervous system. If the parasympathetic supply is interrupted, salivation is severely impaired and the salivary glands atrophy.
Sympathetic fibers to the salivary glands stem from the superior cervical ganglion. Preganglionic parasympathetic fibers travel via branches of the facial and glossopharyngeal nerves (cranial nerves VII and IX, respectively). These fibers form synapses with postganglionic neurons in ganglia in or near the salivary glands. The acinar cells and ducts are supplied with parasympathetic nerve endings.
Parasympathetic stimulation increases synthesis and secretion of salivary amylase and mucins, enhances the transport activities of the ductular epithelium, greatly increases blood flow to the glands, and stimulates glandular metabolism and growth.
Ionic Mechanisms of Salivary Secretion
Fig. 28.3 shows a simplified view of the mechanisms of ion secretion by serous acinar cells. The basolateral membrane of the cell contains Na+,K+-ATPase and an Na+-K+-2Cl− symporter. The concentration gradient for Na+ across the basolateral membrane, which is dependent on Na+,K+-ATPase, provides the driving force for entry of Na+ , K+ , and Cl− into the cell. Cl− and HCO3 − leave the acinar cell and enter the lumen via an anion channel located in the apical membrane of the acinar cell. This secretion of anions drives the entry of Na+ and thus water into the acinar lumen across the relatively leaky tight junctions.
Acinar cell fluid secretion is strongly enhanced in response to elevations in intracellular [Ca++] as a result of activation of the muscarinic receptor for acetylcholine.